For dispensing and pipetting very small liquid amounts in the range of a few pl to μl, micropumps, for example, are utilized as a liquid delivery device, which eject drops through a nozzle of the dispensing or pipetting device. As a micropump, such dispensing or pipetting devices comprise a liquid chamber that contains a sample liquid or a system liquid. A wall of the sample chamber is configured to be, e.g., elastic, particularly as a diaphragm. A pulse generator such as a piezo actuator acts upon the elastic wall. Pressure pulses can be generated in the liquid chamber by the piezo actuator. They cause a delivery of liquid drops from the nozzle. The droplet volume is clearly defined so that the liquid amount delivered is clearly specified via the number of droplets. The volume of the individual drops is in the range of from 50-100 pl.
In principle, micropumps are very liable to fault. This is due to the very small dimensions and the sensitivity to precipitations and gaggings related thereto. Because of its compressibility, the presence of an air bubble, e.g., may result in that no delivery effect or a too small delivery effect occurs even if the diaphragm of the micropump is actuated. Furthermore, surface tension effects may impair the perfect function of such micropumps. Particularly with automation such as, for example, in medium or high throughput screening, a high reliability of such micropumps is required. Malfunctions which might nevertheless occur should be detected and repaired automatically. One problem which often occurs with micropumps, e.g., is that precipitations, e.g. of crystallizations of the liquid in the liquid chamber, are produced within the liquid chamber. Further, gas bubbles, particularly air bubbles, are often produced within the liquid chamber, a channel connected with the liquid chamber or the nozzle. This often results in a failure of the micropump. With micropumps used in dispensing and pipetting devices, gas bubbles at least lead to a falsification of the delivered drop size and thus to a falsification of the delivered liquid amount if they will not directly result in the failure of the micropump.
To avoid malfunctions in such liquid delivery devices, it is known to flush the liquid chamber with liquid in a cleaning step. The liquid in the liquid chamber, i.e., the sample or system liquid, is used for flushing. It is also possible to flush the chamber with flushing liquid via a separate connection of the liquid chamber with a reservoir. If necessary, the pump may be operated at an increased frequency or amplitude. Such known flushing methods, however, are time-consuming. This is particularly disadvantageous if such liquid delivery devices are used in medium or high throughput screening. Further, the liquid consumption in known flushing methods is extremely high. This is particularly disadvantageous when the flushing in a dispensing device has to be done with expensive sample liquid.
It is the object of the invention to provide a method for cleaning a liquid delivery device by means of which a quick and reliable cleaning is guaranteed.